# How long does it take for the Magnetic Field to reach the detector?

• LitleBang
In summary: Maxwell must have been one happy camper! Maxwell must have been one happy camper! Maxwell was very excited when he realized that his theory predicted the speed of light. Maxwell was very excited when he realized that his theory predicted the speed of light.
LitleBang
Suppose I had an electromagnet that would instantly go to full power when turned on (impossible) and at one light second away a detector capable of detecting the electromagnet. When I turn the electromagnet on how long does it take for the field to reach the detector?

One second.

One second.
I am not sure about that because the velocity you are assuming is that of an EM wave. A pure magnetic field does not transfer energy, it just puts it into storage, so I don't think we can measure a propagation delay.

tech99 said:
I am not sure about that because the velocity you are assuming is that of an EM wave. A pure magnetic field does not transfer energy, it just puts it into storage, so I don't think we can measure a propagation delay.
1 s is correct. You cannot switch on a “pure magnetic field”. That action generates an E field too.

davenn
tech99 said:
he velocity you are assuming is that of an EM wave.

Please don't tell me what I am assuming. (Unless you are claiming ESP!) That's what you get using retarded potentials.

tech99 said:
so I don't think we can measure a propagation delay

If you have a magnet and a compass, you most certainly measure the difference in time between energizing the magnet and the motion of the compass.

If you cycle the thing on and off a few hundred thousand times a second, it's a radio, isn't it? Probably not a good one.

anorlunda
Ibix said:
If you cycle the thing on and off a few hundred thousand times a second, it's a radio, isn't it? Probably not a good one.
Yes. In fact a very similar thing happens in MRI. The coils used to transmit RF energy in MRI are actually not designed to transmit in the far field, but predominantly send energy to the local magnetic field, the near field. You can certainly detect far field signals, but as you say they are rather inefficient since most of the energy gets reflected back into the amplifier.

Interesting, a connection to the propagation of light. I can't post new ideas but I can post a question that leads to new ideas. How very very interesting.

The idea that electromagnetic field changes propagate at the speed of light isn't exactly new...

davenn
LitleBang said:
Interesting, a connection to the propagation of light. I can't post new ideas but I can post a question that leads to new ideas. How very very interesting.
As pointed out this is not a new idea. Electromagnetic waves (which is what light is composed of) carry information about changes in the electromagnetic field.

LitleBang said:
Interesting, a connection to the propagation of light.
The history is interesting - here's the very oversimplified version:

That light propagated at a speed of about ##3\times{10}^8## meters/second was known since early in the 18th century (although not necessarily using those units, and ever more accurate measurements have been made since then).
During the 1860s James Maxwell formulated his equations describing the origin and behavior of electrical and magnetic fields, and discovered that these equations predict that changes in the electrical and magnetic fields would produce waves. He calculated the predicted speed of these waves... and the answer turned out to be ##3\times{10}^8## meters/second.
He then somewhat boldly suggested that these waves were light, that light was the electromagnetic radiation that his theory predicted. He was right.

So there is indeed a connection between the propagation of light and the propagation of your magnetic field changes - they are one and the same thing.

Redbelly98, vanhees71 and Ibix
Nugatory said:
He calculated the predicted speed of these waves... and the answer turned out to be ##3\times{10}^8## meters/second.

I have wondered how Maxwell must have felt when he first did that calculation and realized the connection.

## 1. How is the Magnetic Field measured at the detector?

The Magnetic Field is measured using a device called a magnetometer, which detects changes in the strength and direction of the magnetic field.

## 2. What factors affect the time it takes for the Magnetic Field to reach the detector?

The time it takes for the Magnetic Field to reach the detector can be affected by the distance between the source of the field and the detector, the strength of the field, and any obstacles or materials that the field must pass through.

## 3. Can the speed of the Magnetic Field be altered?

No, the speed of the Magnetic Field is constant and cannot be altered. It travels at the speed of light, which is approximately 186,282 miles per second.

## 4. How long does it typically take for the Magnetic Field to reach the detector?

The time it takes for the Magnetic Field to reach the detector can vary greatly depending on the factors mentioned above. In some cases, it can reach the detector in a matter of seconds, while in other cases it may take several minutes or even hours.

## 5. Can the Magnetic Field be detected instantly?

No, the Magnetic Field cannot be detected instantly. It takes time for the field to reach the detector and for the magnetometer to register the changes in the field's strength and direction.

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